This invention relates generally to electromagnetic locks and more particularly to electromagnetic locks of the direct pull type used, for example, to lock a door against a door frame.
In large buildings, it is often desired to maintain certain doors in a locked condition and to control the locking and unlocking thereof from a remote centralized location for security reasons. In some cases, health and safety considerations require that certain doors be unlocked in order to provide ingress and egress during emergency occurrences. For such applications, electromagnetic door locks are particularly well suited. Since they generally require no moving parts, they are not subject to jamming or other mechanical malfunctions which would prevent unlocking them when desired. Locking and unlocking are, thus, easily accomplished using an electrical switch which may be located at a large distance from the door, and which may be manually operated or automatically operated in response to a feedback signal from fire alarm, burglar alarm, or other emergency protective systems.
A direct pull electromagnetic door lock commonly consists of an electromagnet located within a housing which is commonly mounted to a door frame or other stationary structure. An armature, made of a magnetizable material, is mounted to the door, or movable structure, and provides the mechanism by which the electromagnet can grip the door.
One well known method of making the electromagnetic lock housing assembly is to form an elongated rectangular canister having one open longitudinal face. The pieces used to form the canister are welded or otherwise fastened together at all corners. The magnetic lamination stack and coil assembly, the control circuitry, and required bushings are placed within the canister and connections, as appropriate, are made. A potting compound, usually epoxy or other thermosetting resin, is poured into the canister and envelops the components previously inserted This immobilizes all parts with respect to each other and essentially "glues" the parts within the canister. After finishing and testing, the housing assembly is ready for use. Another method for making this assembly is to preform a rectangular parallelepiped of thermosetting resin or other suitable insulating material incorporating the magnetic lamination stack and magnetizing coil wires, placing it within an elongated rectangular parallelepiped canister having one open longitudinal face, and securing it within the canister by means of attachment screws inserted through the canister walls. In this case, the canister itself is held together by fastening screws and is longer than the electromagnetic assembly, thereby providing space within the canister for control circuitry over which a protective cover is fastened.
Both of these methods produce functionally adequate electromagnet housing assemblies. However, in the first case, the unitized structure produced by potting the lamination stack, magnetizing wire coils, and control circuitry within the canister requires scrapping the whole unit in case of even the most minimal defect. In the second case, the screw-fastened assembly of the canister and the retention of the electromagnet subassembly within the canister by threaded fasteners seriously increases the risk of failure of the assembly in service. Very slight loosening of the threaded fasteners in shipping, handling, installation, and service lead to improperly balanced stresses and increase the likelihood of service failures of the electromagnetic housing assembly. Thus, in both cases, assembly of the canister with smooth interior surfaces requires reliance on the gluing effect of the potting compound or the continued uniform fastener tensioning for the mechanical integrity of the electromagnet housing assembly in service.
The armature, an elongated plate of magnetizable material, is fastened to one of the structural members to be locked and coacts with the electromagnet housing assembly to provide the locking function. It is commonly secured to its structural member by a single fastener located at its center. When force is exerted against the locked couple, the armature plate experiences a bending tendency due to concentration of that force at the single central attachment point. Deflection of the plate in response to that force creates a minute gap between the magnet and the plate at the concentration point which drastically reduces the magnetic holding force and which results in substandard lock function.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.